PCI: Enable PCI_DOMAINS along with generic PCI host controller

The generic PCI host controller is often instantiated by hypervisors, and
they may add several of them or add them in addition to a physical host
controller like the Jailhouse hypervisor is doing.  Therefore, allow for
multiple domains so that we can handle them all.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>